Method of casting readily oxidizable metals



Patented Feb. 28, 1933 UNITED STATES PATENT OFFICE ARTHUR W. WINSTON, 0F MIDLAND, MICHIGAN, ASSIGNOR TO THE DOW CHEMICAL COMPANY, OF MIDLAND, MICHIGAN, A CORPORATION OF MICHIGAN METHOD OF CASTING READILY OXIDIZABLE METALS N 0 Drawing.

While the present improvements relate more particularly to the casting of magnesium and so-called light metal alloys, in which magnesium is the predominant constituent, it will be understood that the improved method and means involved is also adaptablefor the casting of other similar readily oxidizable metals. Difiiculties encountered in melting and handling, in molten state, magnesium or an alloy largely composed of this metal are well known, and those difficulties are still present when it is attempted to introduce such metal into a mold in any process of casting. For example, when green sand castings are made in the ordinary Way by simply pouring the molten magnesium or light metal alloy into a water bound mold made with untreated sand, not only is there a pronounced tendency for the metal to burn in the sprue, but the surface of the casting formed by contact with the complementary surface of the mold tends to become oxidized, pitted or both oxidized and pitted due to the formation of a powder (presumably an oxide or a nitride of the metal). Where the casting poured in this manner is of relatively heavy section, it either completely burns to a powder or is badly damaged due presumably to the temperature of the metal cast remain ing at a relatively high point for a longer period of time than is the case with thin section castings which chill promptly. Even if the impairment of the surface of such castings as can be made is not serious enough to prevent the cast article from being finished when removed from the mold, the casting has an unsightly appearance and a certain loss of metal is involved. It is to be noted, of course, that the cleaner the casting, the more easily may the subsequent finishing operations be carried out.

It has heretofore been proposed to overcome these difficulties, specifically in casting magnesium and alloys containing the same, by (lusting the mold prior to casting the metal with sulphur or equivalentmaterial capable of giving off in contact with the molten metal a vapor which is more easily oXidizable than magnesium. This procedure, however, has the objection that it is obvious- Application filed December 20, 1929. Serial No. 415,615.

ly difficult to correctly gauge the amount of sulphur or equivalent material thus used and particularly if an excess is applied to the surface, either of the mold proper or the core, so much vapor may be generated that some of it becomes entrapped during the period that the metal is solidifying, the result being blow-holes or surface-shrinks in the casting.

I have now discovered that unexpectedly satisfactory results in the way of producing a casting with a relatively unimpaired sur-- face may be obtained by intermixing with a molding sand, superficially coating such sand in the mold after the mold is formed, or by a combination of the two with substances distinctly different from sulphur both in their character and apparent behavior. The materials so used may be mixed with or dusted directly on the sand or may be applied thereto in suitable solution or suspension. In general, carbonaceous materials and specifically solid carbonaceous materials may thus be employed with strikingly advantageous results.

To the accomplishment of the foregoing and related ends, the invention, then, consists of the method hereinafter fully de scribed and particularly pointed out in the claims, the following description setting forth in detail several modes of carrying out the invention, such disclosed modes 1llus- 'trating, however, but several of the various Ways in which. the principle of the invention may be used;

The mold which is formed to produce acasting of any desired shape or size, e. g. a

crank case for an internal combustion engine or like article, may be made entirely of green sand treated with solid carbonaceous materials or may be made of ordinary green sand having a facing of substantial thickness composed of sand containing the said materials. In any case the mold may have incorporated therein baked sand cores or metal parts as is well known to the art.

If with the sand employed in making such mold or mold facing approximately 1 to 10 per cent of solid carbonaceous material be intermixed, a very satisfactory result in the way of casting is obtained upon' pouring the metal in the mold in the usual manner, it being noted, of course, that ordinarily unless the casting section is very thin an amount of solid carbonaceous material greater than the minimum noted should be used. Under some conditions and particularly where the casting section is rather thin, relatively satisfactory results may be obtained by dusting or spraying the surface of the finished mold with the said material in dry, solution or suspension form. In some cases it may be preferable to not only mix the carbonaceous material with the sand but also to dust or otherwise apply it to the surface of the finished mold.

Among the carbonaceous materials to be used in the manner heretofore described, the following, e. g. bituminous coal, peat, lignite, coke, pitch, tar, and asphalt, which are solid or semi-solid and other carbon base materials that are solid or semi-solid at room temperature, are satisfactory. These materials all contain a certain amount of volatile matter which apparently aids in protecting the metal from excessive burn1ng.

Where using materials of this type they should be relatively highly divided so as to permit better dispersion in the sand or spread on the same. For convenience all such materials as those noted above will be called hereinafter solid carbonaceous materials.

It is entirely feasible where the mold parts i are treated as hereinbefore described to use the solid carbonaceous materials on or in ordinary dry sand mold.parts such as cores. In any case the use of sand addition agents of the aforementioned carbonaceous group in or on the sand does not preclude the use of sul-' phur or equivalent material, particularly in or around the sprue if found desirable in order to prevent the metal from burning.

It is somewhat difficult to state just how these sand addition agents function and why they make possible the satisfactory casting of readily oxidizable metals. In contradistinction to sulphur, for example, they eombine to a minor extent at casting temperatures with oxygen to form water and oxides of carbon. They all contain more or less complex carbon compounds which form vapors by volatilization. or decomposition or both over a considerable range of temperature. The gases so produced appear to displace, dilute or both displace and dilute the moisture and free oxygen in the molding sand adjacent to the casting and thus prevent excessive burning of the hot metal. It further appears that the carbonaceous material upon being heated by the heat of the metal cast tends to become plastic or semi-plastic, at least in part, so as to seal off a portion of the interstices between the sand particles and thus act somewhat as a barrier to aid in keeping moisture in the green sand from reaching the hot readily oxieoaeoe dizable metal. However, it is to be noted that this is only an attempt to explain why my disclosed sand addition agents prevent excessive burning of the readily oxidizable metal, and accordingly due to the uncertainty as to exactly what happens, I do not wish to be limited to the exact wording of that explanation.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the method herein disclosed provided the step or steps stated by any of the following claims or the equivalent of such stated step or steps be employed.

I therefore particularly point out and distinctly claim as my invention 1. The method of casting a readily oxidizable metal such as magnesium or its alloys so as to eliminate excessive burning thereof which includes the steps of forming a mold having a facing containing a solid carbonaceous material having free carbon and a relatively large proportion of non-aqueous constituents which are volatile at the temperature reached in casting, applying to the surface thereof solid carbonaceous material in finely divided form and then ouring the molten metal in contact therewith.

2. A method of preparing a sand mold part to eliminate excessiveburning of magnesium or its alloys when cast in contact therewith, which comprises admixing 5 to 10 per cent of bituminous coal dust with the molding composition, and forming a mold therefrom.

3. A method of preparing a sand mold part to eliminate excessive burning of magnesium or its alloys when cast in contact therewith, which comprises incorporating one to ten per cent of bituminous coal in finely divided form in such mold part.

4. A method of treating a sand mold part to eliminate excessive burning of a readily oxidizable metal such as magnesium or its alloys when cast in contact therewith, which comprises applying bituminous coal in finely divided form to the surface of such mold part.

5. In a method of eliminating excessive burning of readily oxidizable metal on casting the same in undried molds formed from magnesium and its alloys on casting the same in undried molds, the step which comprises forming a mold from a water bound molding composition wherein said mold has in surface exposure solid carbonaceous material containing free carbon and a relatively large proportion of non-aqueous constituents which are volatile at the temperature reached in casting.

7. In a method of eliminating excessive burning of readily oxidizable metal such as magnesium and its alloys on casting the same in Water bound green sand molds, the step which includes incorporating with said molding sand a solid carbonaceous material having free carbon and a relatively large proportion of non-aqueous constituents which are volatile at the temperature reached in casting, and forming a mold from said molding composition.

Signed by me this 17 day of December,

ARTHUR W. WINSTON. 

